1. Field of the Invention
The present invention relates to a balun transformer, and more particularly, to a balun transformer which can facilitate control of characteristics thereof according to an external condition and reduce energy loss by increasing the strength of electromagnetic coupling.
2. Description of the Related Art
The term ‘balun’ of a balun transformer is short for balance-unbalance. The balun transformer refers to a device that includes a circuit or a structure for converting a balanced signal to an unbalanced signal or vice versa. When a device including a balanced input/output port such as a mixer or an amplifier is connected with a device including an unbalanced input/output port such as an antenna, the balun transformer is used to perform conversion between the balanced and unbalanced signals.
The balun transformer may be realized by combining lumped elements such as resistance-inductance-capacitance (RLC) devices, or as a distributed element such as a micro-strip line, a strip line, and a transmission line. The recent increase in demand for small wireless communication products employing balun transformers increases the use of multilayer balun transformers manufactured using, e.g., low-temperature co-fired (LTCC) technology in order to reduce the product size.
FIG. 1 is an equivalent circuit diagram of a related art balun transformer. The balun transformer includes four conductive signal lines 14 to 17 (hereinafter, also referred to first to fourth lines) each having a length of λ/4 (λ=1/fc where fc denotes a center frequency of an input/output signal). One end of the first line 14 is connected to an unbalanced port 11 receiving or outputting an unbalanced signal of a predetermined frequency, and the other end of the first signal line 14 is connected to one end of the second line 15. The other end of the second line 15 is open. The third and fourth lines 16 and 17, each having one grounded end are disposed parallel to the first and second lines 14 and 15 for electrical coupling, respectively. The other ends of the third and fourth lines 16 and 17 are respectively connected to balanced ports 12 and 13 receiving or outputting balanced signals.
In the above configuration, the first line 14 and the third line 16 form one coupler, and the second line 15 and the fourth line 17 form one coupler. When an unbalanced signal of a predetermined frequency is applied to the unbalanced port 11, electromagnetic coupling occurs between the first and third lines 14 and 16 and between the second and fourth lines 15 and 17. Thus, balanced signals of the same frequency as that of the input unbalanced signal are output through the balanced ports 12 and 13. Here, the balanced signal being output through the balanced ports 12 and 13 have the same amplitude and a phase difference of approximately 180 degrees. In contrast, if balanced signals of a predetermined frequency, which have the same amplitude and a phase difference of approximately 180 degrees are respectively applied to the balanced ports 12 and 13, an unbalanced signal of the same frequency as that of the balanced signal is output from the unbalanced port 11.
As smaller balun transformers are increasingly demanded, research is actively ongoing to further miniaturize the balun transformers while maintaining or improving their fundamental characteristics. In the case of multilayer balun transformers for device miniaturization, their characteristics are affected to a great extent by external conditions. Also, the electronic coupling between couplers decreases in strength, causing energy loss.